Process for the production of ethylene



United States Patent 3,437,714 PROCESS FOR THE PRODUCTION OF ETHYLENEJulian Newman, East Meadow, N.Y., assignor to The Lummus Company, NewYork, N.Y., a corporation of Delaware N0 Drawing. Filed May 21, 1965,Ser. No. 457,863 Int. Cl. C07c 11/04, /00 US. Cl. 260-683 11 ClaimsABSTRACT OF THE DISCLOSURE A process for cracking naphtha to increasethe production of ethylene wherein a feed, containing in addition to thenaphtha, steam and propylene recycled from the cracking process iscracked under the following conditions: a residence time of less thanone second, a heat input of -40M B.t.u./hr. per sq. ft.; and an outletgas temperature of at least 1500 F. The propylene comprises up to of thefeed, exclusive of steam.

This invention relates generally to the conversion of hydrocarbons and,more particularly, to an improved process for producing ethylene fromsuch hydrocarbons. The process of the invention is characterized byhigher conversion rates, lower coking, greater ethylene formation andmore economic operation, including less downtime.

In the cracking of a hydrocarbon feed, such as a crude naphtha for theproduction of ethylene, significant quantities of propylene areunavoidably produced. Where there is no independent market need forpropylene, it has been customary to either burn this fraction of thecracked naphtha as fuel gas, or pass it to a hydrogenation unit. Apropane product is withdrawn from the latter and this is generallycracked in a separate furnace under conditions controlled to produce amaximum amount of ethylene. The recycling of propylene to the naphthacracking furnace has been limited to very small quantities or, in mostcases, avoided altogether, due to the increased coking in the furnacetubes caused thereby, with the resultant shortening of the furnace run.

The rate of conversion of a naphtha feed in a pyrolytic crackingoperation increases with increasing temperature. On the other hand, atthe temperatures where at least a satisfactory conversion rate takesplace, about 1400 F., the reaction products are subject topolymerizaation and condensation reaction, which reactions are highlyexothermic. A short residence time at reaction temperature is thuspreferred. Dilution of the feedstock with steam reduces the hydrocarbonpartial pressure and reduces coke formation, and dilution of about -50%is common practice. Additionally, steam serves to maintain the linearvelocity of the hydrocarbon feed at the maximum for the desiredconversion, thereby reducing the formation of carbon and other secondaryreactions. Rapid quenching to below about 1000 F., outside the furnaceto prevent polymerization reactions, is thus common practice.

The rate of conversion of naphtha indicates how much of the raw materialis reacting, but the reaction products contain a variety of compoundsranging from light ends to tars and aromatics and, as noted above,includes substantial quantities of propylene. Selectivity of thereaction to producing the desired product is very important. Thus, anymethod of cracking which produces more ethylene and less propylene, andwhich avoids the coking problems noted above, will effect substantialeconomies.

It is therefore a general object of the present invention to provide aprocess for conversion of hydrocarbon feeds 3,437,714 Patented Apr. 8,1969 which is generally more efficient and economical than prior artpractices.

Another object of the present invention is to provide a process forproducing ethylene, in which total ethylene produced for a givenconversion rate is greater than has been heretofore possible.

Yet another object of the invention is to provide a process forincreasing the yield of ethylene and decreasing the net propylenewithout increasing formation of coke.

Still another object of the present invention is to provide a processfor producing ethylene by pyrolysis in a cracking furnace whereinsubstantially all of the propylene produced may be recycled directly tothe cracking furnace without rapid coking of the process tubes.

Various other objects and advantages of the invention will become clearin the course of the following description of an embodiment thereof, andthe novel features will be particularly pointed out in connection withthe appended claims.

As noted hereinabove, the efficiency of a given cracking operation isdependent on a number of factors, principal among which are residencetime, outlet gas temperature, heat flux and steam to hydrocarbon ratio.The efficiency of the operation is judged by the ethylene output for agiven energy input.

While one skilled in the art might logically expect that a higher energyinput (i.e., heat flux), shorter residence time and higher outlet gastemperature would increase the conversion rate, he would also expectthermal efficiency and selectivity to decrease.

The present invention is based, at least in part, on the discovery thatwith a very substantially increased heat flux and an even moresubstantial reduction in residence time, the conversion rate increasessomewhat, as would be expected, but contrary to expectations, theselectivity to ethylene is also increased. Moreover, it has beendiscovered that an improved ethylene yield without any increase incoking, and recovery of substantial quantities of benzene, can beachieved by recycling up to of the recovered propylene fraction to thefeed. Thus, recycle propylene, in an amount of up to 25% of the combinedfeed but preferably about 10-20%, will be converted in part to ethyleneand in part to benzene. In this manner, ethylene yields may be increasedup to 10%, and a substantial quantity of benzene recovered. At the sametime, the necessity of a separate hydrogenation unit and crackingfurnace for disposal of the propylene fraction is eliminated.

In brief, the process of the invention may be carried out, for example,with a crude naphtha-propylene mixture introduced into a pyrolysisheater, with a residence time of about 0.4 to 0.6 second, as opposed to2.4 or 2.5 seconds in conventional units. Heat flux to the tubes isabout 20-4OM B.t.u./hr. per square foot of tube surface, as opposed to515M B.t.u./hr. per square foot conventionally employed. Outlet gastemperature is about 100 F. higher than normal, i.e., 1500 to 1600 F.Thus, according to the invention, residence time is about onefifth thatemployed in conventional operations and heat flux is two to six timesgreater. These changes in operating parameters, however, result in botha more efficient and more economic operation. Selectivity to ethylene ata given conversion rate rises. While more fuel is expended according tothe invention, the energy is recoverable to a large extent in heatexchangers following pyrolysis.

Operation in accordance with the invention also yields economics inconstruction and maintenance of the pyrolysis heater. Thus, whereas 'aconventional heater may require a 675 foot process coil, a heater foruse with the present invention may require only a 200 foot coil, as aresult of higher heat flux and shorter residence times involved.

It has been found that, along with increased selectivity to ethyleneproduction, the production of coke precursors is correspondinglydecreased. This feature allows a given furnace to run for longerperiods. While not wishing to be bound to a particular theor ofoperation, it is believed that the higher process gas temperature favorsprimary over secondary reactions, which of course reduces cokeformingreactions. Also, the high temperature favors reaction between coke whichis formed and the dilution steam. It is a significant feature of theinvention that the recycled propylene stream, which increases ethyleneyield and produces benzene, does not appear to encourage coke formation.

Operation at a higher temperature, albeit for longer runs, might beexpected to materially shorten tube life due to the adverse effects ofthermal cycling from the higher temperature on the microstructure of thetube material. While this might be true if conventional tube cleaningprocedures were followed, operation in the 1500 to 1600 F. rangeaccording to the invention makes possible the use of a novel tubecleaning method, which in fact lengthens tube life rather thanshortening it. This procedure forms another aspect of the presentinvention, and comprises the following steps: without extinguishing orturning down the burners in the heater, the feed gas and recycle streamsare turned off, allowing only steam to pass through the heater. Thesteam temperature can be allowed to rise somewhat above 1800 F., and thetemperature at the skin of the metal tube will be within about 100 F. ofthis temperature. At 1800 F.+, the steam-coke reactions will take placerapidly and, after a short period, the tubes will be clean. Of course,during this period, the outlet stream is diverted and the water gasproduced is either vented or recovered, depending on the economies ofthe particular installation. When the process tubes are clean, the feedgas and recycle streams are opened and normal operations resumed. Theadvantage of this type of operation is that the tubes are kept hot on acontinuous basis, and the deleterious effects of thermal cycling ontubes and furnace refractories are avoided. With a plurality of heatersoperating in parallel, continuous heater operation is possible. Decokingis thus reduced to periodic shut-off of the feed and diversion of theproduct stream, and thus becomes a routine operation which may besubjected to automatic (i.e., programmed) control.

It is believed that a better understanding of the invention will begained by referring to the following specific examples thereof, whichare intended to be illustrative only and are not to be interpreted in alimiting sense. For purposes of better illustrating the advantages to begained by the process steps of the present invention, Ex ample Ihereinbelow represents operation without recycling of any fraction,Example II illustrates operation wtih recycling of an ethane fraction,and Example III illustrates operation wherein the process of theinvention is followed.

Example I Five thousand pounds per hour of a light naphtha feedcontaining 90% paraffins and having a 250 F. end point are admixed with2,500 pounds per hour of steam. The resulting mixture is introduced intoa pyrolysis heater having a 250 foot coil (4" ID). The feed is passedthrough the coil at a residence time of 0.5 second and is heated to atemperature of 15S F. to effect pyrolysis of the feed. The effluent fromthe heater was cooled and had the yields 'as set forth in Table I below.

Example II The conditions described in Example I were repeated exceptthat the ethane fraction (Example I) was recycled to a separate heater.The efiiuent from the heater had the yields as set forth in Table 1below.

4 Example III The conditions described in Example I were repeated exceptthat the fractions (Example II) containing propylene and propane wererecycled and admixed with naphtha feed. The efiiuent from the heater hadthe yields as set forth in Table I below.

TABLE I.PYROLYSIS YIELDS (WT. PERCENT) Examples I Components Fuel gas18.0 CgH4 32.0 C2Hs 4. 5 (33116 18. 0 C3Hg 0. 5 C4+ 10. 0 Gasoline 14. 5Fuel oil 2. 5

bons which, when subjected to pyrolysis under conditions to produceprimarily ethylene, will also contain propylene in the effluent from thepyrolysis heater. Thus, in the pyrolysis of propane to produce ethylene,propylene will also be formed during pyrolysis, whereas substantiallylittle is formed during the pyrolysis of ethane to produce ethylene.Additionally, the term is also to be interpreted as including a mixtureof hydrocarbons, such as ethane and propane wherein propylene isincluded in the reactor efiiuent where such mixture is pyrolyzed underconditions to produce ethylene.

It is to be understood that various changes in the details, steps,materials and arrangements of parts, which have herein been described toillustrate the invention, may be made by those skilled in the art withinthe scope of the invention as defined in the appended claims.

What is claimed is:

1. in the production of ethylene from a hydrocarbon feed which uponpyrolysis primarily produces ethylene and some propylene; a process forincreasing ethylene selectivity comprising:

mixing said feed with recycled propylene and dilution steam;

introducing the resulting mixture into a pyrolysis tube;

maintaining a heat input to said mixture within the range of about20,000 to 40,000 b.t.u./hr. per square foot of tube surface, based onoutside tube diameters of 3 to 6 inches;

maintaining said mixture within said tube for no more than one second;

withdrawing a gaseous efiiuent from said tube at a temperature of atleast about 1500 F; cooling said efiluent, including desired ethylene,to a temperature of less than about 1000 F.;

separating ethylene as product from said effluent; and

separating propylene from said effluent and recycling at least a portionthereof to said mixing step.

2. The process as defined in claim 1, wherein said mixture is maintainedwithin said pyrolysis tube for from 0.4 to 0.5 second.

3. The process as defined in claim 1, wherein said recycled propylenecomprises up to 25% of the feed gas, exclusive of dilution steam.

4. The process as defined in claim 1, and additionally comprising theseparation and recovery of benzene from said efiiuent.

5. The process as defined in claim 1, wherein the temperature of saidefiiuent is from about 1,500 P. to 1,650 F. when withdrawn from saidtube.

6. The process as defined in claim 1, wherein naphtha is the hydrocarbonfeed.

7. In the production of ethylene from a hydrocarbon feed which uponpyrolysis primarily produces ethylene and some propylene a continuousprocess for increasing ethylene selectivity comprising:

mixing said feed with dilution steam and recycled propylene;

introducing the resulting mixture into a pyrolysis tube in a pyrolysisheater; maintaining a sufiicient heat flux around said tube to provideabout 20,000 to 40,000 b.t.u./hr. per square foot based on outside tubediameters of 3 to 6 inches;

maintaining said mixture within said tube for about 0.4 to 0.5 second;

withdrawing from said tube a gaseous eflluent, including desiredproducts, at a temperature of about 1,500 F. to 1,650 F.;

cooling said efiluent to a temperature of less than about separating andrecovering in separate fraction, ethylene,

propylene and benzene from said efiluent; and

recycling at least a portion of said propylene to said mixing step.

8. The continuous process as defined in claim 7, and additionallycomprising:

periodically discontinuing the flow of said feed and said recycle gas tosaid tube while maintaining the fiow of said steam;

maintaining within said tube a temperature of at least diverting theefiiuent steam from further process steps;

continuing the passage of steam through said tube until the same iscleaned of coke and any other accretions; and

resuming the flow of said feed and said recycle gas to said tube.

9. In the production of ethylene from a hydrocarbon feed which uponpyrolysis primarily produces ethylene and some propylene; a continuousprocess for increasing ethylene selectivity comprising:

mixing said feed with up to 25% by weight of propylene;

maintaining a sufficient heat fiux around said tube to provide from20,000 to 40,000 b.t.u./hr. per square foot based on outside tubediameters of 3 to 6 inches;

passing the mixture through said tube in a maximum time of one second;and

withdrawing a gaseous efliuent from said tube at a temperature of atleast 1500 F.

10. The process as defined in claim 9, wherein said propylene isrecycled gas, and comprises from 1020% by weight of the hydrocarbonfeed-propylene mixture.

11. The process as defined in claim 9, and additionally comprising theseparation and recovery of ethylene, propylene and benzene as separatefractions, and recycling at least a potrion of the propylene fraction.

References Cited DELBERT E. GANTZ, Primary Examiner.

US. Cl. X.R.

